HART Protocol
The HART (Highway Addressable Remote Transducer) was Developed by Rosemount Inc. as a proprietary digital Communication protocol for smart field instruments in 1986, it was made an open protocol. The HART protocol Provides backward compatible solution for smart instrument Communication as both 4- 20 mA analogue and digital Communication signals are transmitted simultaneously on the Same cable this feature made it the most widely used standard in modern instruments. HART products available now include Analogue and digital process transmitters, Process receivers (valves), local (field) controllers, calibrators, Etc. Overview: HART is intended to allow easy calibration, range Setting, damping adjustment, error control, etc. in Microprocessor based systems. It uses FSK modulation to Transmit digital signals between the devices. Logical 1â„¢s are represented by a frequency of 1.2 kHz and 0â„¢s by a frequency of 2.2 kHz. These signals are then superimposed on to the 4- 20mA control signals of the device. This feature helps in adjusting the operating point of the instrument without affecting its working (on-line calibration) and reduces operation related risk. Fig: FSK signals on 4-20mA analogue control signals The HART communication signals have a frequency range well above the device control signal (about 10 Hz). So simple filters can be used to separate control signals and HART signals. In peer to peer mode only one device is attached to a device loop and both analogue and digital signals are used for communication, while in multi drop mode up to 15 devices are connected to a device loop keeping the analogue signal at 4mA. HART protocol follows a master-slave configuration and allows up to 2 masters (controlling devices like DCS, PLCâ„¢s, handheld HART calibrators, etc.) and many slave devices like temperature and pressure transmitters